In Combat 1 - Launching a Sub you learned how to launch a sub:* Select the outpost you want to send the sub from.* Drag your finger from that outpost to the destination outpost.* Turn the dial to increase the amount of drillers. (Or, when you start to work with specialists later, tap a specialist from the left side to add it to your sub.)

The only difference here, is that in Combat 4, you don't have any drillers on an outpost yet. And you can't send an empty sub. So you'll have to wait until you have an outpost containing some drillers. (The sub containing the drillers will disappear, and the amount of drillers is transferred to the outpost.)